The present invention relates generally to satellite communication systems, and more particularly, to a method for maximizing the downlink information rate by applying adaptive error control coding on the satellite commensurate with the desired quality of the downlink channel.
Satellite-based systems have been employed for many years to provide real-time distribution of information on a global scale. Typically, multiple ground-based users transmit data channels multiplexed into one or more uplink signals to the satellite. A satellite may, for example, demodulate an uplink signal, extract and route the data channels to one of many downlink transmitters, and then remodulate the data channels for transmission on a downlink beam. As another example, a satellite may frequency translate the uplink signal, amplify it, and retransmit it to the ground without demodulation-remodulation.
In commercial applications where capacity generates revenue, one significant performance factor is the amount of information that is passed through the satellite (i.e., throughput). Generally, the higher the data throughput, the higher the revenue potential. However, a significant fraction of the total throughput may not be used for data because of the need to include error-correction coding to reduce the bit error rate ("BER") of the downlink beams. The BER is the ratio of incorrectly received bits to the total number of bits transmitted. Atmospheric gases, rain, clouds, fog, radio noise, depolarization, scintillation, and interference are examples of conditions that may increase the BER. Accordingly, protection is typically applied to the uplink and downlink beams to minimize the BER as seen by the user.
For example, if a user is located in an area of a region experiencing poor transmission or reception conditions (such as an area experiencing a rainstorm), the downlink beam in that region may need more protection because the downlink beam may be susceptible to signal fading (which leads to increased BER) as a result of the rain. In the past, even a small area experiencing increased BER in a larger region required that the downlink beam covering that entire region carry additional error protection.
One way to improve the BER of the downlink data stream is to increase the power level of the transmitted signal. However, this solution is generally undesirable because it is too expensive, power-consuming, and impractical to implement on a large-scale basis. An alternative to increasing the power level of the downlink beam is to channel code the entire data stream (a process known as "coding") before transmitting it to the satellite. Coding the data stream provides added protection to the data stream tantamount to an increase in power level. In particular, one frequently employed method of coding is referred to as "concatenated coding" and was first investigated by G. David Forney, Jr.
Concatenated codes typically employ two levels of coding: an inner and an outer coding. A concatenated code achieves a level of performance with less complexity than a single level of coding would generally require. The outer code typically employs some form of a block code: for example, a Reed-Solomon (RS) code. The block code essentially adds parity bits to each predetermined number of bits (a "block") of data. The inner code typically employs some form of a convolutional code, although block codes are also used.
One class of satellite systems, referred to as end-to-end coded systems, apply all error-control coding (e.g., both inner and outer codes in a concatenated coded scheme) at the uplink ground terminal. In these systems, all encoding and decoding is performed at the ground terminal. At the satellite, the data stream is demodulated, routed to the appropriate downlink transmitters, remodulated, and transmitted to a remote ground terminal. This type of satellite system is commonly referred to as a digital bit-pipe because it is "piping" the data from one ground terminal to another without applying any intermediate coding at the satellite.
Coding is commonly used in satellite systems to reduce transmitter power requirements and overall hardware costs. As disclosed in U.S. Pat. No. 4,800,570 to Perrotta et al., filed Apr. 30, 1987, entitled "Concatenated Code-Decode System for the Protection Against Interference of Digital Transmissions Through an Intermediate Regenerative Repeater," an inner code of a concatenated coding scheme may be applied at the satellite rather than on the ground. In digital telecommunications systems where generating the downlink beam is critical due to the limited transmitter power available on the satellite, applying the inner code of a concatenated coding scheme on the satellite improves the downlink BER without requiring large increases in transmitter power.
In the past, after the transmitted uplink data channels have been recovered to form a data stream on the satellite, the entire data stream is coded in the satellite to achieve a minimum BER for the ground terminal with the worst channel conditions. For example, if users A-Z are served by a given downlink beam and ground terminal A's channel conditions are poor, but ground terminal B-Z's channel conditions are excellent, the entire data stream comprising the downlink beam must nevertheless be coded to achieve a minimum BER for ground terminal A's channel condition. This is generally referred to as non-adaptive error-control coding. Thus, in the past, bandwidth has been wasted by providing ground terminals B-Z with channel coding beyond that actually required.
Applying non-adaptive error-control coding at the satellite is therefore undesirable because it needlessly assumes worst-case channel conditions for all downlink channels and thereby wastes the bandwidth of numerous downlink channels with superfluous error-control coding information. Loss of bandwidth translates into a loss of capacity which can translate into a loss of revenue. Non-adaptive error-control coding also has the further disadvantage of being inflexible. Because the same information rate is transmitted to all ground terminals simultaneously regardless of their channel conditions, users cannot be easily prioritized based on market criteria, such as the amount charged per service and the quality of service.
A need has long been present in the industry for an improved method for maximizing the downlink information rate which overcomes the disadvantages discussed above and previously experienced.